Motor control system and method with predictive motor current estimation

ABSTRACT

A chiller system includes a compressor configured to circulate a refrigerant between an evaporator and a condenser in a closed refrigerant loop and a synchronous motor configured to drive the compressor. The motor includes a stator winding and a rotor. The chiller system includes a controller configured to estimate a flux linkage of the rotor and generate a control signal for the motor based on the estimated flux linkage. Estimating the flux linkage includes applying a voltage of the stator winding to a transfer function having an error correction variable, using a first value of the error correction variable in the transfer function to obtain convergence of the flux linkage over an initial motor starting interval, and using a second value of the error correction variable after the initial motor starting interval to reduce an error in estimating the flux linkage.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/579,464 filed Aug. 16, 2012, which is the U.S. national stage ofInternational Application No. PCT/US2011/026045 filed Feb. 24, 2011,which claims the benefit of and priority to U.S. Provisional PatentApplication No. 61/311,420 filed Mar. 8, 2010. The entire disclosure ofeach of these applications is incorporated by reference herein.

BACKGROUND

The application generally relates to a motor drive for a permanentmagnet motor. The application relates more specifically to a method andsystem for controlling a high speed permanent magnet synchronous motor(PMSM).

Variable speed drives (VSDs) are used to power a variety of motor typesin Heating, Ventilation, Air Conditioning and Refrigeration (HVAC&R)systems. Common types of motors that are used for HVAC&R systems includeinduction motors, switched reluctance motors, and other AC and DC motorscapable of handling the torque and speed ranges required in such HVAC&Rsystems.

Permanent magnet synchronous motors (PMSMs) are of particular interestfor use in HVAC&R systems due to their higher efficiency and higherpower density as compared to regular AC induction motors. PMSMs arerotating electric machines that operate with a permanent magnet rotor. Apermanent magnet rotor may be configured with surface mounted permanentmagnets or with interior permanent magnets having differentconfigurations or arrangements. The stator of a PMSM may be similar to astator of an induction motor. However, a totally different stator designfor a PMSM is possible and stator design optimization may be necessaryeven though the stator topology might be similar to an induction motor.The use of a permanent magnet to generate a substantial air gap magneticflux makes it possible to design highly efficient PMSMs.

A PMSM driven by a sinusoidal current is referred to as a PMSM while, aPMSM driven by a rectangular phase-current waveform can be referred toas a brushless DC (BLDC) machine. The rotor structure of the PMSM andBLDC can be the same as a surface-mounted permanent magnet rotor. Boththe PMSM and BLDC are driven by stator currents coupled with the givenrotor position. The angle between the generated stator flux linkage andthe rotor flux linkage, which is generated by a rotor magnet, definesthe torque, and thus speed, of the motor. Both the magnitude of thestator flux linkage and the angle between the stator flux linkage androtor flux linkage are controllable to maximize the torque or minimizethe losses. To maximize the performance of PMSM and ensure the systemstability, the motor requires a power electronics converter for properoperation.

In order to achieve maximum performance and control when operating aPMSM it is necessary to determine the rotor position. Speed or positionsensors, or a combination of both, can be used to determine the rotorposition. However, speed or position sensors may not perform properlywhen exposed to a harsh environment. The addition of sensors alsoincreases the system cost, and may require a complete disassembly of thePMSM in the event of a sensor failure.

For high speed and ultra-high speed applications, special speed andposition sensors are required, and availability and cost of specialspeed and position sensors may be a problem. Various kinds of sensorlessschemes have been proposed to remove the speed or position sensor byestimating the position from a measured electrical variable, forexample, by obtaining the rotor position information from the fluxlinkage. Accurate flux estimation is required for the rotor positionestimation in the sensorless control of PMSM. The existing methods forflux estimation may be adequate when the ratio of the switchingfrequency to the fundamental frequency is high and the samplingfrequency to fundamental frequency ratio is high. However, whenoperating at a low sampling frequency to fundamental frequency ratio andlow switching frequency to fundamental frequency ratio, which isnormally the case for high speed or ultra-high speed PMSM drives,accurately estimating the flux linkage becomes more difficult. Thus thetraditional methods are not applicable.

Intended advantages of the disclosed systems and/or methods satisfy oneor more of these needs or provide other advantageous features. Otherfeatures and advantages will be made apparent from the presentspecification. The teachings disclosed extend to those embodiments thatfall within the scope of the claims, regardless of whether theyaccomplish one or more of the aforementioned needs.

SUMMARY

In a first embodiment, a method is disclosed for controlling asynchronous motor by determining a rotor position of the synchronousmotor based on estimating a flux linkage. The method includes applying avoltage and current of a stator winding of the motor to a transferfunction. The transfer function includes an S-domain integrationoperation and an error correction variable. The method further includesprocessing an output of the transfer function to compensate for theerror correction variable introduced in the transfer function,generating an estimated rotor flux linkage, computing an angle of therotor position based on the rotor flux linkage and inputting thecomputed rotor position to a controller for controlling a position orspeed of the motor.

In a second embodiment, a chiller system includes a compressor, acondenser, and an evaporator connected in a closed refrigerant loop. Asynchronous motor is connected to the compressor to power thecompressor. A variable speed drive is connected to the motor. Thevariable speed drive is arranged to receive an input AC power at a fixedinput AC voltage and a fixed input frequency and provide an output powerat a variable voltage and variable frequency to the motor. The variablespeed drive includes a converter connectable to an AC power sourceproviding the input AC voltage. The converter is arranged to convert theinput AC voltage to a DC voltage. The variable speed drive furtherincludes a DC link connected to the converter and an inverter connectedto the DC link. The DC link is configured to filter and store the DCvoltage from the converter stage. A controller is arranged to controlrotor speed of the synchronous motor based on an estimated flux linkage.

At least one advantage of the embodiments described herein is a methodto control a high speed surface-mounted PMSM without the need forspeed/position sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary embodiment for a heating, ventilation and airconditioning system.

FIG. 2 shows an isometric view of an exemplary vapor compression system.

FIG. 3 shows schematically an exemplary embodiment for a heating,ventilation and air conditioning system.

FIG. 4 shows schematically an exemplary embodiment of a variable speeddrive

FIG. 5 shows a schematic diagram of an exemplary permanent magnetsynchronous motor.

FIG. 6 is shows schematic diagrams of alternate exemplary rotors of aninternal PMSM (IPM).

FIG. 7 shows an exemplary transfer function for estimating flux in apermanent magnet synchronous motor.

DETAILED DESCRIPTION

FIG. 1 shows an exemplary environment for a heating, ventilation and airconditioning (HVAC) system 10 in a building 12 for a typical commercialsetting. System 10 can include a vapor compression system 14 that cansupply a chilled liquid which may be used to cool building 12. System 10can include a boiler 16 to supply a heated liquid that may be used toheat building 12, and an air distribution system which circulates airthrough building 12. The air distribution system can also include an airreturn duct 18, an air supply duct 20 and an air handler 22. Air handler22 can include a heat exchanger that is connected to boiler 16 and vaporcompression system 14 by conduits 24. The heat exchanger in air handler22 may receive either heated liquid from boiler 16 or chilled liquidfrom vapor compression system 14, depending on the mode of operation ofsystem 10. System 10 is shown with a separate air handler on each floorof building 12, but it is appreciated that the components may be sharedbetween or among floors.

FIGS. 2 and 3 show an exemplary vapor compression system 14 that can beused in HVAC system 10. Vapor compression system 14 can circulate arefrigerant through a circuit starting with compressor 32 and includinga condenser 34, expansion valve(s) or device(s) 36, and an evaporator orliquid chiller 38. Vapor compression system 14 can also include acontrol panel 40 that can include an analog to digital (AID) converter42, a microprocessor 44, a non-volatile memory 46, and an interfaceboard 48. Some examples of fluids that may be used as refrigerants invapor compression system 14 are hydrofluorocarbon (HFC) basedrefrigerants, for example, R-410A, R-407, R-134a, hydrofluoro olefin(HFO), “natural” refrigerants like ammonia (NH₃), R-717, carbon dioxide(CO₂), R-744, or hydrocarbon based refrigerants, water vapor or anyother suitable type of refrigerant.

Motor 50 used with compressor 32 can be powered by a variable speeddrive (DC) power source. Motor 50 can include any type of PMSM that canbe powered by a VSD or directly from an AC or DC power source.

FIG. 4 shows an exemplary embodiment of a VSD. VSD 52 receives AC powerhaving a particular fixed line voltage and fixed line frequency from anAC power source and provides AC power to motor 50 at a desired voltageand desired frequency, both of which can be varied to satisfy particularrequirements. VSD 52 can have three components: a rectifier/converter222, a DC link 224 and an inverter 226. The rectifier/converter 222converts the fixed frequency, fixed magnitude AC voltage from the ACpower source into DC voltage. The DC link 224 filters the DC power fromthe converter 222 and provides energy storage components such ascapacitors and/or inductors. Finally, inverter 226 converts the DCvoltage from DC link 224 into variable frequency, variable magnitude ACvoltage for motor 50.

In an exemplary embodiment, the rectifier/converter 222 may be athree-phase pulse width modulated boost rectifier having insulated gatebipolar transistors to provide a boosted DC voltage to the DC link 224to obtain a maximum RMS output voltage from VSD 52 greater than theinput voltage to VSD 52. Alternately, the converter 222 may be a passivediode or thyristor rectifier without voltage-boosting capability.

VSD 52 can provide a variable magnitude output voltage and variablefrequency to motor 50, to permit effective operation of motor 50 inresponse to a particular load conditions. Control panel 40 can providecontrol signals to VSD 52 to operate the VSD 52 and motor 50 atappropriate operational settings for the particular sensor readingsreceived by control panel 40. For example, control panel 40 can providecontrol signals to VSD 52 to adjust the output voltage and outputfrequency provided by VSD 52 in response to changing conditions in vaporcompression system 14, i.e., control panel 40 can provide instructionsto increase or decrease the output voltage and output frequency providedby VSD 52 in response to increasing or decreasing load conditions oncompressor 32. The estimated rotor phase angle θ_(r) and rotor frequencyω_(r), of motor 50, as described in more detail below, may be input tothe control panel for feedback control of the position and rotationalfrequency of motor 50.

Compressor 32 compresses a refrigerant vapor and delivers the vapor tocondenser 34 through a discharge passage. In one exemplary embodiment,compressor 32 can be a centrifugal compressor having one or morecompression stages. The refrigerant vapor delivered by compressor 32 tocondenser 34 transfers heat to a fluid, for example, water or air. Therefrigerant vapor condenses to a refrigerant liquid in condenser 34 as aresult of the heat transfer with the fluid. The liquid refrigerant fromcondenser 34 flows through expansion device 36 to evaporator 38. A hotgas bypass valve (HGBV) 134 may be connected in a separate lineextending from compressor discharge to compressor suction. In theexemplary embodiment shown in FIG. 3, condenser 34 is water cooled andincludes a tube bundle 54 connected to a cooling tower 56.

The liquid refrigerant delivered to evaporator 38 absorbs heat fromanother fluid, which may or may not be the same type of fluid used forcondenser 34, and undergoes a phase change to a refrigerant vapor. Inthe exemplary embodiment shown in FIG. 3, evaporator 38 includes a tubebundle 60 having a supply line 60S and a return line 60R connected to acooling load 62. A process fluid, for example, water, ethylene glycol,calcium chloride brine, sodium chloride brine, or any other suitableliquid, enters evaporator 38 via return line 60R and exits evaporator 38via supply line 60S. Evaporator 38 lowers the temperature of the processfluid in the tubes. The tube bundle 60 in evaporator 38 can include aplurality of tubes and a plurality of tube bundles. The vaporrefrigerant exits evaporator 38 and returns to compressor 32 by asuction line to complete the circuit or cycle. In an exemplaryembodiment, vapor compression system 14 may use one or more of each ofvariable speed drive (VSD) 52, motor 50, compressor 32, condenser 34,expansion valve 36 and/or evaporator 38 in one or more refrigerantcircuits.

The control method described with respect to FIG. 7 below provides acompensation method for estimating a flux in a permanent magnetsynchronous motor to reduce the estimation error due to the unavoidableDC shift from voltage and current measurements and an error coefficientor correction variable α. The method includes the steps of adjusting thevalue of an error correction variable at selected intervals; changingthe value of error correction variable as a function of a speed of arotor of the PMSM; applying a first value of the error correctionvariable that is configured to obtain faster convergence over apredetermined initial motor starting interval; and applying a secondvalue of the error correction variable after the predetermined initialmotor starting interval, the second value configured to reduce an errorin the flux estimation. In addition, the compensation method reduces theestimation error due to the error correction variable α. An estimatedflux linkage includes a real component alpha and an imaginary componentbeta. The alpha and beta components of the estimated flux linkage may beused to compensate for the error due to the error correction variable α.

Referring to FIG. 5, an exemplary PMSM 86 includes a stator portion 72.The stator portion 72 can be configured substantially similar to astator of a conventional induction motor. Stator portion 72 includes aplurality of windings 74 disposed in slots 25 defined by a plurality ofteeth 17, symmetrically distributed about an inside radius of statorportion 72 adjacent to a rotor portion 70. Rotor portion 70 ispositioned axially concentrically with and inside stator portion 72.Rotor portion 70 and stator portion 72 are separated by an air gap 68.Rotor portion 70 may include a cylindrical steel rotor frame or cage 31with a plurality of permanent magnets 84 arranged peripherally on rotorcage 31. Permanent magnets 84 produce a magnetic field in air gap 68.

Permanent magnets 84 may be positioned or arranged to provide multiplepole arrangements or configurations, for example 2-pole or 4-pole (see,e.g., FIGS. 6A and 6B), in rotor portion 70. Permanent magnets 84 may beadhesively affixed to cage 31, and are enveloped by a sleeve 29 tomaintain permanent magnets 84 on cage 31 when centrifugal forces act onrotor portion 70 during rotation of PMSM 86. Sleeve 29 may beconstructed of carbon fiber tubular sheet material, stainless steel orother similarly flexible, high strength, magnetically non-permeablematerial. Air gap 68 is small relative to an effective air gap g shownbetween opposing arrows 45. Effective air gap g includes the height h ofpermanent magnets 84 and sleeve 29.

Referring next to FIG. 7, in an exemplary method of flux estimation in aPMSM, a low-pass filter is used to obtain the estimated flux. Thetransfer function of the low pass filter is represented by equation 1:

$\begin{matrix}\frac{1}{s + a} & {{Equation}\mspace{14mu} 1}\end{matrix}$

wherein:

s represents an S-domain integration operation of the low-pass filter;and

α=the error correction variable for the low-pass filter.

In one embodiment, the low pass filter represented by the transferfunction may be implemented in software. Alternately, the low passfilter may be implemented in hardware components, e.g., integratedcircuit, ASIC, or R-L-C circuit. By increasing the value of variable αin Equation 1 the convergence of the estimated flux can occur in ashorter time interval, but the error in the estimated flux will begreater. By decreasing the value of the variable α convergence can occurmore slowly, but with a smaller estimation error. For example, whenrotor speed is 94.25 rad/s, a different “α” value yields different angleerrors.

α=2 sin φ=0.9997749 angle error=1.2167°

α=4 sin φ=0.99910058 angle error=2.43025°

α=6 sin φ=0.99797971 angle error=3.64265°

α=8 sin φ=0.99641682 angle error=4.851787°

α=10 sin φ=0.99441813 angle error=6.056610°

An open loop estimation method 100 is shown in FIG. 7. An a-axis voltagev_(a) is applied to an input 101 of a summation block 102. The inputs tothe estimation method are motor phase currents and voltages expressed inα-β coordinate frame. The coordinate frames and voltage and currentvectors of PMSM, being the phase axes a, b and c, and α-axis and β-axisrepresent a fixed Cartesian coordinate frame aligned with phase a;d-axis and q-axis represent a rotating Cartesian coordinate framealigned with rotor flux. The α-β frame expressions for α-axis and β-axisare obtained by applying a Clarke transformation to their correspondingthree phase representation.

An α-axis current value i_(α), representing the phase a stator, orarmature, current, is provided to a current prediction model 104 whichpredicts the current used in the estimation. The output i_(α) of currentprediction model 104 is multiplied by stator resistance r_(s) at block106 to generate an estimated drop-off voltage on stator winding. Theoutput of block 106 is then subtracted from α-axis phase a voltage v_(α)in summation block 102, and the output of block 102 is applied totransfer function block 108, represented by the transfer function ofEquation 1. In transfer function block 108, filter variable α isintroduced to achieve a minimum error at different speed ranges within arequired convergence time. The output of block 108 is then compensatedat block 110 to reduce or remove the error introduced by α in block 108.The output of block 110 is combined at summation block 112 with anoutput of block 114. Block 114 represents the stator mutual inductanceL_(m), which is the value by which the input predicted current i_(α) ismultiplied to generate the output of block 114. The output of block 114is subtracted from the error compensated air gap flux linkage, or outputof block 110, at block 112. The difference of blocks 110, 114 is theoutput of block 112, representing the estimated rotor flux linkage inα-axis. Then the rotor phase angle θ_(r) is estimated by anglecalculation block 116 using the estimated rotor flux linkage from both αand β-axis. The estimated rotor phase angle θ_(r) 120 is applied toblock 118, and the time domain derivative of the estimated rotor phaseangle θ_(r) 120 is generated at the output of block 117 as the rotorfrequency ω_(r).

More precise flux estimation and control of PMSM 86 may be achieved byadjusting the value of variable α at selected intervals. For example,the value of α may be changed as a function of the rotor speed whereinthe value of α is greater initially, i.e., upon starting PMSM 86 toobtain a faster convergence at the beginning of the estimation. Afterthe convergence, a smaller value of α is applied to reduce the error inthe flux estimation. In addition, in an exemplary embodiment, the valueof α may be transitioned gradually, or ramped down from the higherinitial value of α to the lower value of α, to provide improved systemstability.

An exemplary embodiment of the method is described as follows:

ramp down α:

If (ω_(r)>1200 and ω_(r)<1500)

α=5−(ω_(r)−1200)*0.01

If (ω_(r)>1500)

α=2

(“α”=error correction variable)

A compensation method is disclosed to reduce the estimation error due toα. The estimated flux linkage includes two components, a real componentalpha (α) and an imaginary component beta (β). Alpha and beta componentsof the estimated flux linkage may be used to compensate for themagnitude error due to the error correction variable α. The expressionsof the magnitude compensation are given by the following equations:

$\begin{matrix}\frac{\sqrt{\omega_{r}^{2} + a^{2}}}{\omega_{r}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

wherein:

α is the error correction variable and

ω_(r) is the rotor speed.

The second step is for the phase angle correction, in which the fluxlinkage complex variable includes two components, a real component alphaand an imaginary component beta, is used. Alpha and beta components ofthis complex variable may be used to compensate for the phase error dueto the error correction variable α. The expressions of the complexvariable used for phase angle correction is given as:

$\begin{matrix}{\frac{\omega_{r}}{\sqrt{\omega_{r}^{2} + a^{2}}} - {j\frac{a}{\sqrt{\omega_{r}^{2} + a^{2}}}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

If the magnitude compensation and the phase compensation are combined,the expressions of the compensation are given by the followingequations:

$\begin{matrix}{f_{\alpha}^{\prime} = {f_{\alpha} - {f_{\beta}\frac{a}{\omega_{r}}}}} & {{Equation}\mspace{14mu} 4} \\{f_{\beta}^{\prime} = {f_{\beta} \mp {f_{\alpha}\frac{a}{\omega_{r}}}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

wherein:

f′_(α) and f′_(β) are the alpha and beta components of the flux linkageafter compensation;

f_(α) and f_(β) are the alpha components of the flux linkage beforecompensation; and

ω_(r) is the rotor speed.

Since the ratio of the sampling frequency to the fundamental frequencyis low, a relatively long delay is unavoidable without increasing thesampling frequency. Increasing the sampling frequency generallyincreases the cost of a control system. A current prediction method canbe employed to predict the current to eliminate the effect of thesampling delay, without the need for increasing the sampling frequency.For high speed and ultra high speed applications, the sampling numberper cycle becomes greatly reduced. To reduce the estimation error, thecurrent at the N^(th) sampling interval may be used to predict thecurrent at the next (N+1)^(th) sampling interval. The predicted currentmay then be used to reduce the error in the flux linkage estimation. Thecurrent prediction is based on the PMSM machine model.

An exemplary embodiment of the method is described.

The prediction method is based on the q-d reference:

Predicted q axis current is given as:

$i_{qs}^{N + 1} = {i_{qs}^{N} + {T_{s}\frac{\left( {V_{qs}^{N} - {r_{s}i_{qs}^{N}} - {\omega_{r}\lambda_{ds}^{N}}} \right)}{L_{s}}}}$

predicted d-axis current is given as:

$i_{ds}^{N + 1} = {i_{ds}^{N} + {T_{s}\frac{\left( {V_{ds}^{N} - {r_{s}i_{ds}^{N}} + {\omega_{r}\lambda_{qs}^{N}}} \right)}{L_{s}}}}$

wherein,

V_(qs) ^(N) and V_(ds) ^(N) are the q- and d-axis voltages at the Nthsampling interval;

i_(qs) ^(N) and i_(ds) ^(N) are the q- and d-axis currents at the Nthsampling interval;

i_(qs) ^(N+1) and i_(ds) ^(N+1) are the predicted q- and d-axis currentsat the (N+1)^(th) sampling interval;

λ_(qs) ^(N) and λ_(ds) ^(N) are the q- and d-axis flux linkages at theN^(th) sampling interval;

ω_(r) is the rotor speed.

While the foregoing describes a control system and method forcontrolling a sensorless PMSM, the control system and method forestimating an angle position and speed of a motor may be applied toother types of sensorless, synchronous motors, e.g., induction-typemotors, and such synchronous motors are considered to be within thescope of control system herein described and claimed.

It should be understood that the application is not limited to thedetails or methodology set forth in the following description orillustrated in the figures. It should also be understood that thephraseology and terminology employed herein is for the purpose ofdescription only and should not be regarded as limiting.

While the exemplary embodiments illustrated in the figures and describedherein are presently preferred, it should be understood that theseembodiments are offered by way of example only. Accordingly, the presentapplication is not limited to a particular embodiment, but extends tovarious modifications. The order or sequence of any processes or methodsteps may be varied or re-sequenced according to alternativeembodiments.

The present application contemplates methods, systems and programproducts on any machine-readable media for accomplishing its operations.The embodiments of the present application may be implemented using anexisting computer processors, or by a special purpose computer processorfor an appropriate system, incorporated for this or another purpose orby a hardwired system.

It is important to note that the construction and arrangement of theflux estimation method for PMSM control as shown in the variousexemplary embodiments is illustrative only. Although only a fewembodiments have been described in detail in this disclosure, those whoreview this disclosure will readily appreciate that many modificationsare possible (e.g., variations in sizes, dimensions, structures, shapesand proportions of the various elements, values of parameters, mountingarrangements, use of materials, colors, orientations, etc.) withoutmaterially departing from the novel teachings and advantages of thesubject matter recited in the application. For example, elements shownas integrally formed may be constructed of multiple parts or elements,the position of elements may be reversed or otherwise varied, and thenature or number of discrete elements or positions may be altered orvaried. Accordingly, all such modifications are intended to be includedwithin the scope of the present application. The order or sequence ofany process or method steps may be varied or re-sequenced according toalternative embodiments. Other substitutions, modifications, changes andomissions may be made in the design, operating conditions andarrangement of the exemplary embodiments without departing from thescope of the present application.

As noted above, embodiments within the scope of the present applicationinclude program products comprising machine-readable media for carryingor having machine-executable instructions or data structures storedthereon. Such machine-readable media can be any available media that canbe accessed by a general purpose or special purpose computer or othermachine with a processor. By way of example, such machine-readable mediacan comprise RAM, ROM, EPROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage or other magnetic storage devices, or anyother medium which can be used to carry or store desired program code inthe form of machine-executable instructions or data structures and whichcan be accessed by a general purpose or special purpose computer orother machine with a processor. When information is transferred orprovided over a network or another communications connection (eitherhardwired, wireless, or a combination of hardwired or wireless) to amachine, the machine properly views the connection as a machine-readablemedium. Thus, any such connection is properly termed a machine-readablemedium. Combinations of the above are also included within the scope ofmachine-readable media. Machine-executable instructions comprise, forexample, instructions and data which cause a general purpose computer,special purpose computer, or special purpose processing machines toperform a certain function or group of functions.

It should be noted that although the figures herein may show a specificorder of method steps, it is understood that the order of these stepsmay differ from what is depicted. Also two or more steps may beperformed concurrently or with partial concurrence. Such variation willdepend on the software and hardware systems chosen and on designerchoice. It is understood that all such variations are within the scopeof the application. Likewise, software implementations could beaccomplished with standard programming techniques with rule based logicand other logic to accomplish the various connection steps, processingsteps, comparison steps and decision steps.

What is claimed is:
 1. A chiller system comprising: a compressorconfigured to circulate a refrigerant between an evaporator and acondenser in a closed refrigerant loop; a synchronous motor configuredto drive the compressor, the motor comprising a stator winding and arotor; and a controller configured to estimate a flux linkage of therotor by: measuring an electric current of the stator winding at a firstsampling interval; predicting an electric current of the stator windingat a second sampling interval as a function of the measured electriccurrent at the first sampling interval; and using the predicted electriccurrent to estimate the flux linkage of the rotor at the second samplinginterval; wherein the controller is configured to provide a controlsignal to the motor based on the estimated flux linkage.
 2. The chillersystem of claim 1, wherein the controller is configured to predict theelectric current of the stator winding at the second sampling intervalusing a current prediction model.
 3. The chiller system of claim 1,wherein the controller is configured to predict the electric current ofthe stator winding at the second sampling interval as a function of aflux linkage of the rotor at the first sampling interval.
 4. The chillersystem of claim 1, wherein the controller is configured to use thepredicted electric current to reduce a flux linkage estimation error. 5.The chiller system of claim 1, wherein the controller is configured toestimate the flux linkage of the rotor at the second sampling intervalby: measuring a voltage of the stator winding; estimating a drop-offvoltage of the stator winding at the second sampling interval as afunction of the predicted electric current at the second samplinginterval; and using a difference between the measured voltage and theestimated drop-off voltage of the stator winding at the second samplinginterval to estimate the flux linkage of the rotor at the secondsampling interval.
 6. The chiller system of claim 5, wherein thecontroller is configured to estimate the drop-off voltage of the statorwinding at the second sampling interval by multiplying the predictedelectric current at the second sampling interval by a resistance of thestator winding.
 7. The chiller system of claim 5, wherein the controlleris configured to estimate the flux linkage of the rotor at the secondsampling interval by: applying the difference between the measuredvoltage and the estimated drop-off voltage of the stator winding as aninput to a transfer function; and evaluating the transfer function toestimate the flux linkage of the rotor at the second sampling interval.8. The chiller system of claim 1, wherein the controller is configuredto predict the electric current of the stator winding at the secondsampling interval by: determining a q-axis component of the measuredelectric current and a d-axis component of the measured electric currentat the first sampling interval; using the q-axis component of themeasured electric current at the first sampling interval to predict aq-axis component of the predicted electric current at the secondsampling interval; and using the d-axis component of the measuredelectric current at the first sampling interval to predict a d-axiscomponent of the predicted electric current at the second samplinginterval.
 9. The chiller system of claim 8, wherein the controller isconfigured to predict each of the q-axis component and the d-axiscomponent of the predicted electric current at the second samplinginterval using: an axis-specific component of a voltage of the statorwinding at the first sampling interval; and an axis-specific componentof the estimated flux linkage at the first sampling interval.
 10. Thechiller system of claim 1, wherein the controller is configured to:determine a position of the rotor based on the estimated flux linkage;and generate the control signal for the motor based on the position ofthe rotor.
 11. A method for controlling a synchronous motor having astator winding and a rotor, the method comprising: measuring an electriccurrent of the stator winding at a first sampling interval; predictingan electric current of the stator winding at a second sampling intervalas a function of the measured electric current at the first samplinginterval; using the predicted electric current to estimate a fluxlinkage of the rotor at the second sampling interval; providing acontrol signal to the motor based on the estimated flux linkage; andusing the control signal to operate the motor by adjusting a speed orposition of the motor based on the control signal.
 12. The method ofclaim 11, wherein predicting the electric current of the stator windingat the second sampling interval comprises applying the measured electriccurrent at the first sampling interval as an input to a currentprediction model.
 13. The method of claim 11, wherein the electriccurrent of the stator winding at the second sampling interval ispredicted as a function of a flux linkage of the rotor at the firstsampling interval.
 14. The method of claim 11, further comprising usingthe predicted electric current to reduce a flux linkage estimationerror.
 15. The method of claim 11, wherein estimating the flux linkageof the rotor at the second sampling interval comprises: measuring avoltage of the stator winding; estimating a drop-off voltage of thestator winding at the second sampling interval as a function of thepredicted electric current at the second sampling interval; and using adifference between the measured voltage and the estimated drop-offvoltage of the stator winding at the second sampling interval toestimate the flux linkage of the rotor at the second sampling interval.16. The method of claim 15, wherein estimating the drop-off voltage ofthe stator winding at the second sampling interval comprises multiplyingthe predicted electric current at the second sampling interval by aresistance of the stator winding.
 17. The method of claim 15, whereinestimating the flux linkage of the rotor at the second sampling intervalcomprises: applying the difference between the measured voltage and theestimated drop-off voltage of the stator winding as an input to atransfer function; and evaluating the transfer function to estimate theflux linkage of the rotor at the second sampling interval.
 18. Themethod of claim 11, wherein predicting the electric current of thestator winding at the second sampling interval comprises: determining aq-axis component of the measured electric current and a d-axis componentof the measured electric current at the first sampling interval; usingthe q-axis component of the measured electric current at the firstsampling interval to predict a q-axis component of the predictedelectric current at the second sampling interval; and using the d-axiscomponent of the measured electric current at the first samplinginterval to predict a d-axis component of the predicted electric currentat the second sampling interval.
 19. The method of claim 18, whereineach of the q-axis component and the d-axis component of the predictedelectric current at the second sampling interval is predicted using: anaxis-specific component of a voltage of the stator winding at the firstsampling interval; and an axis-specific component of the estimated fluxlinkage at the first sampling interval.
 20. The method of claim 11,further comprising: determining a position of the rotor based on theestimated flux linkage; and generating the control signal for the motorbased on the position of the rotor.